The present invention relates to a steel which exhibits improved formability and thermal fatigue resistance and which is particularly advantageous for use in exhaust manifolds of automobiles.
An exhaust manifold for an exhaust system of an automobile is exposed to high temperature exhaust gas discharged from an internal combustion engine. A material for use in making exhaust manifolds is required to be superior in many characteristics, such as oxidation resistance, high temperature strength, and thermal fatigue resistance.
Conventionally, cast iron has been used for making exhaust manifolds. Recently in order to improve engine performance as well as fuel mileage by decreased weight, welded pipes of stainless steel after shaping have been used as exhaust manifolds. An exhaust manifold made of stainless steel pipe can be 30-40% lighter than one made of cast iron.
However, typical stainless steels containing 16-18% of Cr (SUS 430 Series, ferritic stainless steels) do not exhibit a satisfactory level of oxidation resistance and high temperature strength, and they cannot be used to manufacture exhaust manifolds capable of withstanding a temperature of 900.degree. C. or higher. Austenitic stainless steels containing 18% of Cr and 8% of Ni (SUS 304 Series) have a large thermal expansion coefficient and are easily fractured by thermal fatigue caused by thermal strains introduced when they are subjected to a repeated cycle of heating and cooling.
In view of thermal fatigue resistance and material costs, it is concluded that ferritic stainless steels are preferred to austenitic stainless steels as a material for use in making exhaust manifolds.
Japanese Patent Application Unexamined Laid-Open Specification No.64-8254/1989 discloses ferritic stainless steels containing 17-20% of Cr and 1.0% or less of Mo which are advantageous in making exhaust manifolds exhibiting improved high temperature oxidation resistance and high temperature strength.
However, the above-mentioned publication does not suggest anything about thermal fatigue characteristics, which are most important in the performance of exhaust manifolds exposed to a high temperature atmosphere at 900.degree. C. or higher.